1. Field of the Invention
The present invention relates to color printing, and in particular relates to a fast, iterative method for deriving a look-up table that represents the color mapping for a printer from device independent colors (such as a color in CIEXYZ or CIELAB color space) to a color in a device dependent color space (such as a color in CMY or CMYK color space).
2. Background of the Invention
In conventional computer systems that print color images on a color printer, the precise colors actually printed by the printer are calculated by the computer using a look-up table. Specifically, the look-up table is arranged as a three-dimensional grid of cells, with each entry in the grid representing printer colorant values (such as cyan, magenta, yellow and black colorant values) as a function of some desired color in a different (usually device independent) color space. Based on a desired color, the computer accesses the look-up table to determine the printer colorant values. For colors not stored exactly in the grid, interpolation is conventionally employed so as to determine the colorant values.
Recently, there has been increased interest in producing printed color images with superior color fidelity, such that the printed image matches a displayed image or a scanned-in image. To accomplish color matching, it has become customary to employ a two-step procedure. In the first step, the scanned-in or displayed image, which typically is stored in device dependent color space such as RGB color space, is transformed into a device independent color space such as CIEXYZ or CIELAB color space. This first transformation allows for compensation and calibration of device dependent characteristics, such as phosphor spectral characteristics or spectral sensitivity characteristics of a scanner. In the second step, the device independent colors are transformed into printer device dependent colors, such as CMY or CMYK colors. This second transformation allows for compensation of printer characteristics.
One difficulty with this approach is the determination of entries for the look-up table that gives the transformation from device independent colors to device dependent colors for the printer. In the past, this look-up table was derived once at the factory, based on empirical measurements of a wide variety of color patches printed at fixed colors in the device dependent color space. A single derivation of a look-up table, however, does not allow for post-factory compensation based on effects such as printer aging, selection of different inks with different spectral characteristics, selection of different print media, and other effects which change printout characteristics. Although it is possible to recalibrate the printer by printout of new color patches and by empirical measurement of the resulting device independent colors, it is still difficult, tedious and time consuming to derive a new printer look-up table, even though such derivation is ordinarily performed by a computer.